Chemistry 331

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Chemistry 331
Lecture 13 Enthalpy
NC State University
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Motivation
The enthalpy change DH is the change in energy at
constant pressure. When a change takes place in
a system that is open to the atmosphere, the
volume of the system changes, but the pressure
remains constant. In any chemical reactions that
involve the creation or consumption of molecules
in the vapor or gas phase there is a work term
associated with the creation or consumption of
the gas.
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Molar Enthalpy
Enthalpy can be expressed as a molar
quantity We can also express the relationship
between enthalpy and internal energy in terms of
molar quantities For an ideal or perfect gas
this becomes Usually when we write DH for a
chemical or physical change we refer to a molar
quantity for which the units are kJ/mol.
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Enthalpy for reactions involving gases
If equivalents of gas are produced or consumed in
a chemical reaction, the result is a change in
pressure-volume work. This is reflected in the
enthalpy as follows. which can be rewritten for
an ideal gas The number of moles n is the
number of moles created or absorbed during the
chemical reaction. For example,
CH2CH2(g) H2(g) CH3CH3(g) Dn
-1 We arrive at this value from the formula
Dn nproducts - nreactants 1 - 2
-1
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The temperature dependence of the enthalpy change
Based on the discussion the heat capacity from
the last lecture we can write the temperature
dependence of the enthalpy change as Note that
we can use tabulated values of enthalpy at 298
K and calculate the value of the enthalpy at any
temperature of interest. We will see how to use
this when we consider the enthalpy change of
chemical reactions (the standard enthalpy
change). The basic physics of all temperature
dependence is contained in the above equation or
more frequently in the equation below as molar
quantity
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Another view of the heat capacity
At this point it is worth noting that the
expressions for the heat capacity at constant
volume and constant pressure can be related to
the temperature dependence of U and
H, respectively. The heat capacity is the
rate of change of the energy with temperature.
The partial derivative is formal way of
saying this.
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The heat capacity is also a function of
temperature
We have treated the heat capacity as a constant
up to this point. That is a valid approximation
under many circumstances, but only over a
limited range of temperature. In the general case
the temperature dependence of the enthalpy can be
described as The parameters a, b, and c are
given in Tables. Actually, this expression is
readily integrated in the general case to give
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Enthalpy of physical change
A physical change is when one state of matter
changes into another state of matter of the same
substance. The difference between physical and
chemical changes is not always clear, however,
phase transitions are obviously physical changes.
Fusion
Vaporization
Freezing
Condensation
Sublimation
Vapor Deposition
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Properties of Enthalpy as a State Function
The fact that enthalpy is a state function is
useful for the additivity of enthalpies. Clearly
the enthalpy of forward and reverse processes
must be related by so that the phase changes
are related by Moreover, it should not
matter how the system is transformed from the
solid phase to the gas phase. The two processes
of fusion (melting) and vaporization have the
same net enthalpy as sublimation.
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Question
Which is statement is false A. DsubH gt 0 B.
DcondH lt 0 C. DfusH gt 0 D. DvapH lt 0
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Addivity of Enthalpies
Because the enthalpy is a state function the same
magnitude must be obtained for direct conversion
from solid to gas as for the indirect conversion
solid to liquid and then liquid to gas. Of
course, these enthalpies must be measured at the
same temperature. Otherwise an appropriate
correction would need to be applied as described
in the section on the temperature dependence of
the enthalpy.
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Question
Which statement is true? A. DsubH DfusH -
DvapH B. DvapH DsubH - DfusH C. DfusH DsubH
DvapH D. DvapH DsubH DfusH
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Chemical Change
In a chemical change the identity of substances
is altered during the course of a reaction. One
example is the hydrogenation of ethene
CH2CH2(g) H2(g) CH3CH3(g) DH -137
kJ The negative value of DH signifies that the
enthalpy of the system decreases by 137 kJ and,
if the reaction takes place at constant pressure,
137 kJ of heat is released into the
surroundings, when 1 mol of CH2CH2 combines
with 1 mol of H2 at 25 oC.
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Standard Enthalpy Changes
The reaction enthalpy depends on conditions (e.g.
T and P). It is convenient to report and tabulate
information under a standard set of
conditions. Corrections can be made using heat
capacity for variations in the temperature.
Corrections can also be made for variations in
the pressure. When we write DH in a
thermochemical equation, we always mean the
change in enthalpy that occurs when the
reactants change into the products in their
respective standard states.
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Standard Reaction Enthalpy
The standard reaction enthalpy, DrH , is the
difference between the standard molar enthalpies
of the reactants and products, with each term
weighted by the stoichiometric coefficient.
The standard state is for reactants and
products at 1 bar of pressure. The unit of
energy used is kJ/mol. The temperature is not
part of the standard state and it is possible to
speak of the standard state of oxygen gas at 100
K, 200 K etc. It is conventional to report
values at 298 K and unless otherwise specified
all data will be reported at that temperature.
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Enthalpies of Ionization
The molar enthalpy of ionization is the enthalpy
that accompanies the removal of an electron from
a gas phase atom or ion H(g)
H(g) e-(g) DH 1312 kJ For
ions that are in higher charge states we must
consider successive ionizations to reach that
charge state. For example, for Mg we have
Mg(g) Mg(g) e-(g)
DH 738 kJ Mg(g) Mg2(g)
e-(g) DH 1451 kJ We shall show that
these are additive so that the overall enthlalpy
change is 2189 kJ for the reaction
Mg(g) Mg2(g) 2e-(g)
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Electron Gain Enthalpy
The reverse of ionization is electron gain. The
corresponding enthalpy is called the electron
gain enthalpy. For example
Cl(g) e-(g) Cl-(g) DH -349
kJ The sign can vary for electron gain.
Sometimes, electron gain is endothermic. The
combination of ionization and electron gain
enthalpy can be used to determine the enthalpy of
formation of salts. Other types of processes
that are related include molecular dissociation
reactions.
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Enthalpies of Combustion
Standard enthalpies of combustion refer to the
complete combination with oxygen to carbon
dioxide and water. For example, for methane we
have CH4(g) 2O2(g) CO2(g) 2H2O(l)
DcH -890 kJ Enthalpies of combustion are
commonly measured in a bomb calorimeter (a
constant volume device). Thus, DUm is measured.
To convert from DUm to DHm we need to use the
relationship DHm
DUm DngasRT The quantity Dngas is the change in
the stoichiometric coefficients of the gas phase
species. We see in the above express that
Dngas -2. Note that H2O is a liquid.
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Question
Fill in the missing stoichiometric coefficients
for the combustion reaction C5H12(g)
XO2(g) YCO2(g) ZH2O(l) A. X4, Y8,
Z12 B. X8, Y5, Z6 C. X4, Y10, Z6 D. X2,
Y1, Z6
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Question
Fill in the missing stoichiometric coefficients
for the combustion reaction C5H12(g)
8O2(g) 5CO2(g) 6H2O(l) A. X4, Y8,
Z12 B. X8, Y5, Z6 C. X4, Y10, Z6 D. X2,
Y1, Z6
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Question
Determine Dngas for the reaction as written
C5H12(g) 8O2(g) 5CO2(g) 6H2O(l) A.
Dngas 3 B. Dngas 8 C. Dngas -4 D. Dngas
-3
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Question
Determine Dngas for the reaction as written
C5H12(g) 8O2(g) 5CO2(g) 6H2O(l) A.
Dngas 3 B. Dngas 8 C. Dngas -4 D. Dngas
-3 DHm DUm
DngasRT The quantity Dngas is the change in the
stoichiometric coefficients of the gas phase
species. We see in the above express that
Dngas -2. Note that H2O is a liquid.
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Question
What is the work term for expansion against the
atmosphere? C5H12(g) 8O2(g) 5CO2(g)
6H2O(l) A. DngasRT B. DUm DngasRT C. DUm -
DngasRT D. Dngas
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Question
What is the work term for expansion against the
atmosphere? C5H12(g) 8O2(g) 5CO2(g)
6H2O(l) A. DngasRT B. DUm DngasRT C. DUm -
DngasRT D. Dngas
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Hesss Law
We often need a value of DH that is not in the
thermochemical tables. We can use the fact that
DH is a state function to advantage by using
sums and differences of known quantities to
obtain the unknown. We have already seen a
simple example of this using the sum of DH of
fusion and DH of vaporization to obtain DH of
sublimation. Hesss law is a formal statement
of this property. The standard enthalpy of a
reaction is the sum of the standard enthalpies of
the reactions into which the overall reaction may
be divided.
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Question
Consider the reactions
H(g) H(g) e-(g) DH 1312
kJ Cl(g) e-(g) Cl-(g)
DH -349 kJ Which statement is
true about the charge transfer from H to Cl to
form H and Cl-? A. DH 963 kJ B. DH 1661
kJ C. DH 1312 kJ D. DH -349 kJ
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Question
Consider the reactions
H(g) H(g) e-(g) DH 1312
kJ Cl(g) e-(g) Cl-(g)
DH -349 kJ Cl(g)
H(g) Cl-(g) H(g) DH 963
kJ Which statement is true about the charge
transfer from H to Cl to form H and Cl-? A. DH
963 kJ B. DH 1661 kJ C. DH 1312 kJ D. DH
-349 kJ
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Question
Consider the reactions
H(g) H(g) e-(g) DH 1312
kJ Cl(g) e-(g) Cl-(g)
DH -349 kJ What further
information do you need to calculate the enthalpy
for the reaction H2 Cl2 2H(aq)
2Cl-(aq)? A. DH of ionization and DH of electron
capture B. DH of formation, DH of dissociation
and DH of solvation C. DH of ionization and DH of
solvation D. DH of dissociation and DH of
solvation
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Question
Consider the reactions
H(g) H(g) e-(g) DH 1312
kJ Cl(g) e-(g) Cl-(g)
DH -349 kJ What further
information do you need to calculate the enthalpy
for the reaction H2 Cl2 2H(aq)
2Cl-(aq)? A. DH of ionization and DH of electron
capture B. DH of formation, DH of dissociation
and DH of solvation C. DH of ionization and DH of
solvation D. DH of dissociation and DH of
solvation
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Application of Hesss Law
We can use the property known as Hesss law to
obtain a standard enthalpy of combustion for
propene from the two reactions C3H6(g) H2(g)
C3H8(g) DH -124
kJ C3H8(g) 5O2(g) 3CO2(g) 4H2O(l) DH
-2220 kJ If we add these two reactions we
get C3H6(g) H2(g) 5O2(g) 3CO2(g)
4H2O(l) DH -2344 kJ and now we can
subtract H2(g) 1/2O2(g) H2O(l)
DH -286 kJ to obtain C3H6(g)
9/2O2(g) 3CO2(g) 3H2O(l) DH -2058 kJ
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Variation in Reaction Enthalpy with Temperature
Since standard enthalpies are tabulated at 298 K
we need to determine the value of the entropy at
the temperature of the reaction using heat
capacity data. Although we have seen this
procedure in the general case the calculation
for chemical reactions is easier if you start by
calculating the heat capacity difference between
reactants and products and then substitute
this into the expression If the heat
capacities are all constant of the
temperature range then
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Standard Enthalpies of Formation
The standard enthalpy of formation DfH is the
enthalpy for formation of a substance from its
elements in their standard states. The reference
state of an element is its most stable form at
the temperature of interest. The enthalpy
of formation of the elements is zero. For
example, lets examine the formation of water.
H2(g) 1/2 O2(g) H2O(l) DH
-286 kJ Therefore, we say that DfH (H2O, l)
-286 kJ/mol. Although DfH for elements in their
reference states is zero, DfH is not zero for
formation of an element in a different
phase C(s, graphite) C(s, diamond)
DfH 1.895 kJ/mol
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Question
Consider the formation of carbon dioxide at 298
K C(s) O2(g)
CO2(g) How would you find the heat of
formation of oxygen? A. Look up DfH for C(s) and
subtract it from that of CO2. B. Look it up the
standard thermodynamic tables. C. The heat of
formation of O2 is zero by definition. D. It is
equal to the standard bond energy of two
oxygen atoms.
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Question
Consider the formation of carbon dioxide at 298
K C(s) O2(g)
CO2(g) How would you find the heat of
formation of oxygen? A. Look up DfH for C(s) and
subtract it from that of CO2. B. Look it up the
standard thermodynamic tables. C. The heat of
formation of O2 is zero by definition. D. It is
equal to the standard bond energy of two
oxygen atoms.
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Question
Consider the formation of carbon dioxide at 150
K C(s) O2(g)
CO2(g) How would you find the heat of
formation of CO2? Apply a correction to the
enthalpy from A. Hesss law B. the van der
Waals equation of state C. ideal gas law D. none
of the above
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Question
Consider the formation of carbon dioxide at 150
K C(s) O2(g)
CO2(g) How would you find the heat of
formation of CO2? Apply a correction to the
enthalpy from A. Hesss law B. the van der
Waals equation of state C. ideal gas law D. none
of the above